Quantitation scale for acid-fast bacillus smears

according to stain used

Carbol fuchsin

(X 1000)

Fluorochrome

(X 250)

Quantity

reported

No AFB/3 fields No AFB/30 fields No AFB seen

1–2 AFB/300 fields 1–2 AFB/30 fields Doubtful; repeat test

1–9 AFB/100 fields 1–9 AFB/10 fields Rare (1+)

1–9 AFB/10 fields 1–9 AFB/field Few (2+)

Contd...

Microbiology and Bacteriology 853

1–9 AFB/fields 10–90 AFB/field Moderate (3+)

>9 AFB/fields >9 AFB/field Numerous (4+)

However, Indian Reference Institutions recommend

reporting after 5 minutes of examination covering about

100 fields. Grading is done as follows:

Number of Acid Fast

Bacilli observed

Report

No Acid Fast Bacilli Negative

1–10 Acid Fast Bacilli Actual Number

>10 Acid Fast Bacilli +

Masses of Acid Fast + +

Bacilli in several fields

Smears with fewer than 3 AFB per slide account for about

85% of false positive smear reporting and are considered

doubtful. A repeat specimen should be registered. However,

Mycobacterium tuberculosis infection must be considered

for any patient with repeat smear AFB positive regardless

of the number of AFB observed.

Factors Influencing Sensitivity and Specificity

of AFB Smears

False Positive Results

Acid Fast Particles Other Than Tubercle Bacilli

Occasionally, a sputum specimen or smear may contain

particles that are acid-fast, i.e. when treated with the

Ziehl-Neelsen method, they retain the red stain (carbol

fuchsin) and resist decolorization with acid-alcohol. These

red particles may sometimes resemble tubercle bacilli.

They include certain food particles (e.g. waxes, oils),

precipitates,